The present invention is directed to devices used on local-area networks. It is directed particularly to providing such devices with means for detecting short circuits on the networks.
A local-area network is a medium of communication among a plurality of data-processing or data-communication devices. A popular type involves the use either of a single coaxial cable to which various network devices are connected or of a plurality of cable sections connected in series by repeaters. In communicating over the cable, the devices observe a "carrier sense, multiple access with collision detection" (CSMA/CD) protocol. According to the protocol, only a single device at a time is allowed to transmit information over the network. Sometimes more than one device at a time attempts to initiate transmission of an information packet, but the CSMA/CD protocol provides a mechanism by which the various devices determine which device will transmit its packet.
The various devices on a local-area network may be physically separated by large distances; they are often in separate buildings. Moreover, human control over the system may be somewhat distributed, too, in that different people may be adding and removing devices without the immediate knowledge or control of the local-area-network manager. Under such conditions, short circuits in the cable can cause considerable inconveniences.
Suppose, for instance, that a user installs a device that is defective in such a way that it shorts out the cable. A typical result is that some symptoms of a collision occur. Devices attempting to transmit accordingly stop transmission and then start again, but they keep encountering collisions. System software typically responds with messages that indicate that the devices cannot complete attempted transmissions, and network operation grinds to a halt. The person in charge of the network does not know that the failure occurred when the defective device was added, and he does not know what kind of problem is causing the failure, so he is forced to engage in a significant amount of trouble-shooting before he can identify the problem. In the interim, the network is inoperative not only on the actual shorted cable section but on the other cable sections as well. That is, the problem can be identified as a short circuit only after the network is shut down.
An object of the present invention is to reduce the difficulty that results from a short circuit in a local-area-network cable. Another object is to identify a defect as a short circuit in some circumstances even while the local-area network is still operating.